Site-specific endonucleases theoretically allow for the targeted manipulation of a single site within a genome, and are useful in the context of gene targeting as well as for therapeutic applications. In a variety of organisms, including mammals, site-specific endonucleases, for example, zinc-finger nucleases (ZFNs), have been used for genome engineering by stimulating either non-homologous end joining or homologous recombination. In addition to providing powerful research tools, ZFNs also have potential as gene therapy agents, and two ZFNs have recently entered clinical trials: one, CCR5-2246, targeting a human CCR-5 allele as part of an anti-HIV therapeutic approach (NCT00842634, NCT01044654, NCT01252641), and the other one, VF24684, targeting the human VEGF-A promoter as part of an anti-cancer therapeutic approach (NCT01082926).
Precise targeting of the intended target site is crucial for minimizing undesired off-target effects of site-specific nucleases, particularly in therapeutic applications, as imperfect specificity of some engineered site-specific binding domains has been linked to cellular toxicity. However, the site preferences for engineered site-specific nucleases, including current ZFNs, which cleave their target site after dimerization, has previously only been evaluated in vitro or in silico using methods that are limited to calculating binding and cleavage specificity for monomeric proteins.
Therefore, improved systems for evaluating the off-target sites of nucleases and other nucleic acid cleaving agents are needed and would be useful in the design of nucleases with better specificity, especially for therapeutic applications.